1. Field of the Invention
This invention relates to a low speed brushless electric motor and more particularly to a stepper type motor having a speed reduction mechanism as an integral part of its stator-rotor construction.
2. Description of Related Art
Electric motors generally have a high speed rotor that is connected to an output shaft. For many applications, a separate speed reduction unit, in the form of gear boxes, belts, pulleys, traction drives, etc., is required to deliver an operating speed consistent with the speed and torque requirements of the application.
Speed reduction units separate from the driving motor using trochoidal contours have been widely used U S. Pat. Nos. 1,682,563; 3,998,112; 4,487,091; 4,584,904; and 4,643,047 describe more or less typical speed reduction units. In each of these units, a high speed rotary input drives an inner orbital rotor, having a perimeter defined by an epitrochoidal contour. This rotor is surrounded by a ring having either a number of spaced rollers or a hypotrochoidal interior contour. In either case the number of rollers or lobes on the stator-ring is either one or two greater than the number of lobes on the orbiting rotor. Typically, the rotor is mounted for free rotation on an eccentric keyed to the input shaft. Rotation of the shaft causes the rotor to move orbitally and to rotate with a speed reduction that is a function of the number of lobes on the rotor. In each case, the speed reduction unit is separate from the driving source.
When speed is reduced in the conventional manner it is difficult to start stop or reverse the output because of the high inertia of the rotor and other parts of the system. Moreover, when such a system is stalled because of over loading or other factors, the motor may over heat or even be destroyed. Attempts have been made to alleviate some of these problems with very light weight rotors, but the inherent requirement for a high-speed rotor continue to make sudden starts, stops or reversals a difficult operation requiring in many instances an excessive time period.